MAX3675 Просмотр технического описания (PDF) - Maxim Integrated

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MAX3675

622Mbps, Low-Power, 3.3V Clock-Recovery and Data-Retiming IC with Limiting Amplifier

Maxim Integrated 

622Mbps, Low-Power, 3.3V Clock-Recovery

and Data-Retiming IC with Limiting Amplifier

Reduced Power Consumption

Without the Limiting Amplifier

The limiting amplifier is biased independently from the

clock recovery circuitry. Grounding INSEL turns off the

limiting amplifier and selects the PECL DDI inputs.

Converting Average Optical Power

to Signal Amplitude

Many of the MAX3675’s specifications relate to input-

signal amplitude. When working with fiber optic

receivers, the input is usually expressed in terms of

average optical power and extinction ratio. The rela-

tions given in Table 2 and Figure 6 are helpful for con-

verting optical power to input signal when designing

with the MAX3675.

In an optical receiver, the input voltage to the limiting

amplifier can be found by multiplying the relationship in

Table 2 by the photodiode responsivity and transim-

pedance amplifier gain.

Optical Hysteresis

Power and hysteresis are often expressed in decibels.

By definition, decibels are always 10log (power). At the

inputs to the MAX3675 limiting amplifier, the power is

VIN2/R. If a receiver’s optical input power (x) increases

by a factor of two, and the preamplifier is linear, then the

voltage at the input to the MAX3675 also increases by a

factor of two.

The optical power increase is 10log(2x / x) = 10log(2) =

+3dB.

At the MAX3675, the voltage increase is:

( ) 10log 2VIN 2 / R = 10log(22) = 20log(2) = + 6dB

VIN2/ R

Table 2. Optical-Power Relations*

PARAMETER SYMBOL

RELATION

Average

Power

PAVE

PAVE =(P0 + P1) / 2

Extinction

Ratio

re

Optical Power

of a “1”

P1

Optical Power

of a “0”

P0

Signal

Amplitude

PIN

re = P1 / P0

P1 =

2PAVE

re

re + 1

( ) P0 = 2PAVE / re +1

( ) PIN = P1− P0 = 2PAVE

re − 1

re + 1

*Assuming a 50% average input data duty cycle.

In an optical receiver, the dB change at the MAX3675

always equals 2x the optical dB change.

The MAX3675’s typical voltage hysteresis is 3.0dB. This

provides an optical hysteresis of 1.5dB.

Jitter in Optical Receivers

Timing jitter, edge speeds, aberrations, optical disper-

sion, and attenuation all impact the performance of

high-speed clock recovery for SDH/SONET receivers

(Figure 7). These effects decrease the time available

for error-free data recovery by reducing the received

“eye opening” of nonreturn-to-zero (NRZ) transmitted

signals.

P1

PAVE

P0

TIME

Figure 6. Optical Power Relations

EYE DIAGRAM WITH NO TIMING JITTER

MIDPOINT

TIME

MIDPOINT

EFFECTS OF TIMING JITTER ON EYE DIAGRAM

TIME

Figure 7. Eye Diagram With and Without Timing Jitter

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